Polyvinyl chloride, and various types of polyolefins, especially polyethylenes, are typical polymers used in diverse pipe applications, including transportation of gas such as natural gas, and liquids including domestic cold and hot water, waste water and sewers, as well as uses for water solar systems, floor and radiant heating systems. Cross-linked polyethylene (PEX) pipes are used extensively in portable cold and hot water and domestic and commercial heating applications.
Cross-linked polyethylene pipes, PEX, offer a number of advantages compared to those made from non-cross-linked materials including good heat deformation resistance, increased mechanical strength, stiffness, hardness, and improved abrasion and environmental stress crack resistance, and good high temperature strength.
However, under extreme conditions of use, even PEX pipes have the opportunity for further improvement. For example for domestic portable water, particularly in hot (up to 90° C.) water distribution applications, pipes are subjected to various thermal stresses resulting in creation of severe extractive environment and reduced thermal oxidative resistance. Further potential problems for polyolefin pipes are associated with the presence of oxidising agents, other than oxygen of the environment, such as chlorine and hypochlorous acid which are typically used as disinfectants in water treatment to prevent the spread of infectious diseases. These oxidants can cause oxidative damage to the polymer resulting in possible early brittle fracture of the pipe.
Reduction of oxidative stability of plastics pipes therefore presents a potential problem in extreme conditions and is a decisive factor that is exacerbated in the presence of elevated temperatures, particularly under load, and extractive conditions, and this has a direct effect on reducing the service lifetime of the product.
It would be advantageous therefore to increase the thermo-oxidative stability of the piping material in order to protect against oxidative degradation by providing the polymer with increased processing (melt) stability and long term thermal stability during the lifetime of the hot water pipes which are typically designed for a 50 years lifetime including a significant safety factor.
In order to protect against oxidative degradation and improve the long term stability of polymer products including PEX pipes, they are generally stabilised by stabilisers and antioxidants which are typically incorporated during the manufacture of polymer products. Stabilisers (also known as antioxidants) are a group of compounds containing certain antioxidant functional groups (antioxidant functions) that are capable of interrupting the oxidative degradation process either by deactivation of the damaging free radicals, or by preventing or inhibiting the generation of initiating free radicals especially from peroxide decomposition. Examples of such antioxidant functions which may be utilised in the present invention are chain breaking antioxidant functions, notably hindered phenol, hindered aliphatic amine and aromatic amine functions, peroxide decomposing antioxidant functions including a variety of compounds containing sulfur- or phosphorous-functions, hydroxybenzophenones, transition metal-containing compounds, metal deactivating agents such as disalicylidine ethylene diamine.
However, it is known in the state of the art, that the use of stabilisers/antioxidants in cross-linked polymers, particularly in PEX, presents limitations and challenges. The concentration of antioxidants has been shown to be lower after cross-linking compared to their concentration before cross-linking resulting in decrease of product long term stability. The process of cross-linking interacts with the stabilisers/antioxidants resulting in the antioxidants being, at least partly, deactivated or consumed by the cross-linking process thus an adverse effect on long term stability and service life of the products.
Migration, leaching and loss of antioxidants/stabilisers from polyolefin polymers and to some extent PEX pipes when in contact with fluids such as hot water and other extractive liquids present another problem which can result in a decrease in the stabilising effectiveness of the antioxidant, embrittlement and premature failure of the product as well as giving rise to potential toxicity and hygiene issues when the product is involved in human-contact applications such as the case for pipes targeted for domestic portable water.
More generally, whilst the long term stability of a plastic material can easily be raised to high level, i.e. >100 min in an oxidative induction time (OIT) test, by simply adding larger amounts of stabiliser, this is not a viable technique when manufacturing goods that will be in contact with foodstuffs or portable water pipes because most stabilisers are known to easily migrate out of the matrix, especially when in touch with hot water. Thus, unnecessarily high stabiliser content can be harmful because the stabilisers themselves or their reaction products can have health issues when being diffused out of the system. Hence the amount of stabilisers should be minimised so that migration is reduced or prevented, if possible. Nevertheless, depending on the nature of the antioxidant (that is, the extent/level of its harmful effect), even modest concentrations of antioxidant can give rise to health issues due to migration.
Returning to the industrially important PEX materials, cross-linking of polyethylene is achieved by one of three methods: peroxide (PEXa), silane technology (PEXb) or electron beam (PEXc). Cross-linking of polymers is mainly a radical reaction which applies to most cross-linking processes.
The PEXa approach or “peroxide” method, involves typically the use of special high pressure extruders where chemical cross-linking of the polyethylene in the presence of a peroxide is achieved in the polymer melt during the high temperature manufacturing process. An example of a manufacturing method used for the PEXa process is the Engel method which provides good control of the degree of cross-linking resulting in a uniform product. Typically the degree of cross-linking in PEXa products is in the range 75-85%.
Examples of the PEXb approach are the Sioplas® and Monsil® methods, both of which are used commercially. In these methods, a secondary, post-extrusion process, is used so that cross-linking occurs after extrusion with the help of a catalyst and exposure of pipe to water (e.g. steam or hot water). In this way, cross-linking is provided by —Si—O—Si— bonds between the hydrocarbon polymer chains. Suitably the piping is flushed with water after cross-linking to remove contaminants. Typically the degree of cross-linking is in the range 65-70%.
An example of the third approach, PEXc, is electron irradiation, also known as “cold” cross-linking. Typically the tubing is bombarded with electrons after it has been extruded. In practice, the PE is extruded in the normal way and then moved to an E-beam facility and passed under an electron beam in an irradiation chamber where it is dosed with a controlled amount of radiation resulting in the cross-linking process. Typically the degree of cross-linking is in the range 70-75%.
A number of additional problems have been observed with these polyethylene cross-linking systems: in the peroxide process, peroxide residues are typically left in the polymer system and this could adversely affect polymer stability if pipe is not processed correctly; the electron beam process causes both cross-linking and chain scission which can be detrimental to stability as well as properties of the polymer.